1. Field of the Invention
The present invention relates, in general, to dielectric heating, and in particular, to the use of radio-frequency power to dry dielectric materials in a continuous process by heating the dielectric materials.
2. Description of Related Art
Hygroscopic plastic resins must be voided of moisture before they can be melted and processed into finished products, as by injection molding machines, extruders, and other well-known means. Moisture, if allowed to remain within the resins, would result in a degradation of physical and visual properties of the resulting end products that are formed from the processed resins.
The conventional method of drying hygroscopic resins is by placing the resin material, usually in pellet form, into a hopper and then passing dry, heated air through the bed of resin. The air heats and dries the resin, but only after an extended processing period typically lasting from two to eight hours, depending on the particular resin material and the amount of initial moisture that must be removed. The resin material typically flows through the hopper at a rate dependent upon the consumption of the downstream injection molding machine or extruder, and the hopper is sized, in a manner well-known to those skilled in the art, so as to provide the necessary residence time of the resin in the hopper to achieve the desired moisture removal.
It is well-known that the use of dielectric heating, such as by microwaves or other radio-frequency energy, will greatly enhance the drying of resins and reduce the required drying time by a factor of five to ten times. Several Japanese companies have developed and marketed expensive microwave dryers, but these prior art microwave dryers only operate in batch or semi-batch modes, making such dryers unsuitable for continuous processes. It is therefore desirable to have a practical apparatus and continuous process that provides the necessary residence times for moisture removal drying together with uniform and controlled dielectric heating to accomplish the drying.
Heretofore, radio-frequency transmission lines have been used in dielectric heating apparatus to transfer electromagnetic energy from a generator source to an applicator load that, in turn, heats dielectric material placed within an electromagnetic field created within the applicator. It is well-known to model such a transmission line as distributed capacitance, inductance and resistance, and an electromagnetic field will be created along the transmission line that will transfer energy from the generator source to the applicator load most efficiently when the transmission line has certain proper characteristics. Some important characteristics to be considered are the impedance of the generator source, the terminating impedance of the applicator load, the length of the transmission line, and the characteristic impedance ("Z") of the transmission line. The characteristic impedance of a transmission line is a function of the distributed capacitance and inductance per unit length of the line, and is generally also a function of the oscillation frequency of the electromagnetic energy traveling down the transmission line.
For a transmission line whose length is equal to a quarter-wavelength (i.e., .lambda./4) of the electromagnetic energy traveling down the transmission line, transmission line theory teaches that, when such a line is terminated by either a short circuit or an open circuit, the electrical characteristics of such a transmission line are the same as that of a resonant parallel inductor-capacitor circuit. Such a resonant parallel circuit is often used as a "tank" circuit driven by an electromagnetic generator source to determine the oscillation frequency of the generator source.
The use of such transmission lines by the prior art to transfer electromagnetic energy from a generator source to an applicator load has several disadvantages. First, electromagnetic power is lost in the transference of electromagnetic energy down the feeding transmission line because such transmission lines are not lossless, thereby causing prior art devices to be inefficient. Second, as different materials, each having differing dielectric constants, are placed into the applicator load, heretofore known heating apparatus require re-adjustment of various circuit elements, so as to accommodate the changes in load impedance, in order to maintain efficient power transfer from the generator source to the applicator load. Third, conventional applicator heating apparatus are known to require multiple feed points of radio-frequency energy when the path of the heated dielectric material through the applicator becomes long.
It is therefore desirable to have an apparatus and method for efficiently transferring electromagnetic power from a generator source into heated dielectric materials without the power losses heretofore seen in transmission lines connecting the generator source to its applicator load. It is further desirable to provide an apparatus and method for heating dielectric materials such that no re-adjustment of circuit elements is required as the dielectric constant of the dielectric materials changes. Finally, it is still further desirable to provide an apparatus and method for heating dielectric materials that allow the use of a much longer applicator length, for a given oscillation frequency, than heretofore possible without requiring multiple feed points of radio-frequency energy.